Re: [equinox-dev] DS and bundle stop/start

[BJ Hargrave <hargrave@xxxxxxxxxx>]

Why doesn't DS just asynchronously process
bundles which are lazy activated (not lazy started which is an incorrect
term)? Then you have the same behavior (async processing) regardless of
whether the bundle is lazily or eagerly activated.
--

BJ Hargrave Senior Technical Staff Member, IBM OSGi Fellow and CTO of the OSGi Alliance hargrave@xxxxxxxxxx

office: +1 386 848 1781 mobile: +1 386 848 3788

From:	John Arthorne <John_Arthorne@xxxxxxxxxx>
To:	equinox-dev@xxxxxxxxxxx
Date:	2009/10/26 14:32
Subject:	[equinox-dev] DS and bundle stop/start
Sent by:	equinox-dev-bounces@xxxxxxxxxxx

---

I came across an interesting problem today involving DS and expicitly starting/stopping
bundles. After chatting with Tom he suggested I raise it here for general
awareness and to discuss whether the behaviour makes sense.

In various places in p2 today we explicitly start bundles for various reasons.
We typically use Bundle.start(Bundle.START_TRANSIENT) for this purpose.
We are starting to use DS in p2 today, and we have a few places where a
bundle acquires a service that it registered via DS in its own BundleActivator.start
method. It turns out that DS only processes/starts service components synchronously
for bundles that are lazy started. If you start a bundle explicitly the
DS processing occurs asynchronously, and as a result the services provided
via DS are not available at the time the bundle starts. The result is subtlely
different bundle behaviour (or outright failures), if a bundle is started
explicitly via Bundle#start versus implicitly via lazy activation:

1) Lazy start case: 
 a) bundle's service component is processed and services provided by the
component are registered by DS 
 b) bundle activator starts, and can use services registered in 1a)

2) Activation via Bundle.start(Bundle.START_TRANSIENT):
  a) bundle's activator starts, and services are not yet available
  b) bundle's service component is processed and services provided
by the component are registered by DS 

It turns out there is a coding pattern that can be used to make the explicit
start case match the lazy start case: 

                    
   final
Bundle bundle = ...;//get some bundle 
                    
   bundle.start(Bundle.START_ACTIVATION_POLICY);
                    
   bundle.start(Bundle.START_TRANSIENT);

The call to start(Bundle.START_ACTIVATION_POLICY) causes DS to process
the bundle and register its component services, but does not start the
bundle. The second call to start with Bundle.START_TRANSIENT actually starts
the bundle. 

The moral of the story seems to be that we need to use this "double
start" coding pattern anywhere we are starting bundles, because those
bundles might be using DS and relying on the activation order. Or, perhaps
someone has a suggestion for changes that can be made to the framework
or DS so that these cases behave consistently... 

John_______________________________________________
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